1. Field of the Invention
The present invention relates to a method and device for determining the acceptability of backgate-effect characteristics in semiconductor elements fabricated with wafers as a base.
2. Description of the Background Art
FETs (field-effect transistors) and HEMTs (high-electron mobility transistors) are capable of high-speed switching, and RF Amplifying, and therefore are currently being put to use in a variety of devices such as mobile-phone amplifiers and switches. High operability is consequently demanded of FETs and HEMTs as working devices, and backgate-effect characteristics are measured as one indicator of the precision of the components.
Mention of the backgating effect is made in Japanese Pub. Pat. H06-077425. Applying a negative voltage to the wafer reverse face, on the opposite side of the wafer surface that is formed with FETS, captures at a deep level electrons implanted through the wafer reverse face into the substrate, and due to the captured electrons, elevates the electrostatic potential of the wafer reverse-face side. The result in a situation in which the substrate is furnished with n-channel FETs is that the depletion layer extends out from its side at the reverse face of the wafer to the channel side of the n-channel FET, narrowing the channel and leading to reduction in the drain current. This phenomenon is called the backgate effect; semiconductor circuit elements in which the backgate effect is small are desired.
After being completed as semiconductor circuit elements, FETs and HEMTs to date have been measured for backgate-effect characteristics to judge their acceptability as semiconductor elements.
Nevertheless, inasmuch as backgate-effect characteristics of semiconductor circuit elements such as FETs and HEMTs are determined after they are fabricated up until the product final stage, in those instances in which the backgate characteristics are unacceptable the time and effort required in fabricating the FETs and HEMTs will be wasted, and moreover, a great number of defective products will end up being produced.
An object of the present invention, which came about in order to resolve the issues noted above, is through a method and device for determining backgate characteristics to enable preventing the fabrication of semiconductor circuit elements in which the backgate characteristics are defective.
The present inventors, making a concerted study in order to resolve the foregoing issues, brought to light the fact that when the C-V characteristics of a wafer are to be sought, by applying a separate voltage to the reverse face of the wafer and comparing with the situation in which voltage is not applied to the reverse face, a change in capacitance is evident. They discovered furthermore that this change in wafer capacitance correlates with the backgate characteristics of semiconductor components such as FETs and HEMTs. The present invention came about on the basis of this correlation.
A backgate-characteristics determination method under the present invention is characterized by including a step of finding a first C-V characteristic indicating a relation between voltage applied to the obverse face of a wafer that is to be a semiconductor circuit-element substrate, and capacitance of the wafer; a step of finding a wafer second C-V characteristic while applying a voltage to the reverse face of the wafer; and a step of determining backgate-effect characteristics for the semiconductor circuit elements by comparing the first C-V characteristic and the second C-V characteristic of the wafer.
Initially, the value of the voltage applied to the wafer obverse face is varied, and the first C-V characteristic is found by measuring capacitance at each of the applied voltage values. Next, while applying a given voltage to the reverse side of the wafer, the value of the voltage that is applied to the wafer obverse face is varied likewise as is the case with the first C-V characteristic, and a second C-V characteristic is found by measuring the capacitance at each of the applied voltage values. Then the backgate characteristics of semiconductor circuit elements fabricated with that wafer as a base are determined by comparing the first and the second C-V characteristics based on the above-mentioned correlation. In this way finding the C-V characteristics of the wafer prior to fabricating the semiconductor circuit elements enables predicting the acceptability of backgate characteristics of semiconductor components such as FETs and HEMTs fabricated utilizing the wafer.
In the present invention, by representing the abovementioned first C-V characteristic as a first C-V curve and representing the abovementioned second CV characteristic as a second C-V curve, the backgate-effect characteristics are preferably determined based on a voltage-shift amount between the first and second C-V curves. Representing the C-V characteristics as C-V curves enables determination of the backgate characteristics to be made visually.
It is further preferable that the abovementioned voltage-shift amount be made the difference between a voltage value that the first C-V curve represents for a predetermined capacitance value, and a voltage value that the second C-V curve represents for that capacitance value, and that the backgate characteristics be determined based on such voltage-shift amount. The fact that in finding the second C-V characteristic a given voltage-shift amount can accordingly be found merely by seeking that voltage value at which the capacitance is a predetermined value enables the backgate characteristics to be readily determined.
Further preferable is determining that the backgate-effect characteristics are unacceptable when the above-noted voltage-shift amount is a predetermined value or more, and that the backgate-effect characteristics are acceptable when it is less than the predetermined value. Establishing a predetermined value for voltage-shift amount based on empirical data makes the determination standard clear, thereby making for enhanced reliability of the determination.
It is preferable in the present invention to form on the wafer obverse face at least two Schottky electrodes that differ in surface area and, applying a voltage between such Schottky electrodes, find the abovementioned first and second C-V characteristic. Schottky electrodes can be obtained simply by contacting a metal on the obverse face of the wafer, which is a semiconductor, to lend them form, and electrodes therefore may be readily formed on the wafer obverse face.
It may be had, furthermore, that a Schottky electrode and an ohmic electrode are formed on the wafer obverse surface, and voltage is applied between the Schottky electrode and the ohmic electrode. Inasmuch as having the one of the electrodes be an ohmic electrode lessens the series resistance, more accurate measurements may be made.
The foregoing Schottky electrodes preferably are formed by a metal masking process in which a metal sheet, through which a hole is bored only in a portion that forms the abovementioned Schottky electrode, is contacted onto the obverse face of the wafer and metal is vapor-deposited thereon. The Schottky electrodes thus can readily be formed.
The foregoing Schottky electrodes are preferably formed by a photolithographic process. Vapor-depositing metal by transferring/developing a pattern formed in a photomask (pattern master plate) onto the wafer using an exposure device makes it possible to form the electrodes with high precision in their pattern.
Further preferable is utilizing as the foregoing Schottky electrodes an electrolyte capable of etching the wafer. C-V characteristics sought in a layer in the wafer interior can be thereby be found.
Utilizing a liquid metal as the foregoing Schottky electrode is also preferable. Voltage can be readily applied to the wafer surface in this case because the electrode can be formed just by contacting the liquid metal on the obverse face of the wafer.
It is preferable that a metal layer be formed on the reverse face of the abovementioned wafer by vapor deposition, and that the abovementioned second C-V characteristic be found by bringing a voltage-applying terminal of a power source into contact with such metal layer. The resultant superficial condition of the voltage-applying terminal makes it possible to control the contact resistance against fluctuating, which makes it possible to apply voltage accurately to the entirety of the wafer reverse face.
The above-noted second C-V characteristic preferably is found by directly contacting the voltage-applying terminal of the power source against the reverse face of the abovementioned wafer. Because in this case there is no need to form an electrode on the reverse face, voltage can readily be applied to the reverse face of the wafer.
It is further preferable that the abovementioned wafer be made of GaAs and that an alloy containing Au and Ge as base materials be vapor-deposited onto the reverse face of such wafer. Voltage can thereby be readily applied to the reverse face of the wafer by contacting the voltage-applying terminal on such alloy.
A backgate-characteristics determination device under the present invention is characterized in being equipped with a first power source for applying voltage to the obverse face of a wafer that is to be a semiconductor circuit-element substrate, a C-V gauge for measuring C-V characteristics representing a relation between voltage applied by the first power source and capacitance of the wafer, and a second power source for applying voltage to the reverse face of the wafer; and in that such C-V gauge measures a first C-V characteristic of the wafer in a state in which voltage is not applied by the second power source, and a second C-V characteristic of the wafer in a state in which voltage is applied by the second power source.
Initially, the value of the voltage applied to the wafer obverse face by the first power source is varied, and the first C-V characteristic is found by measuring capacitance using the C-V gauge at each of the applied voltage values. Next, while applying a given voltage using the second power source to the reverse side of the wafer, the value of the voltage that is applied to the wafer obverse face using the first power source is varied likewise as is the case with the first C-V characteristic, and a second C-V characteristic is found by measuring the capacitance using the C-V gauge at each of the applied voltage values. Then the acceptability of backgate characteristics of working devices fabricated with that wafer as a base are determined by comparing the first and the second C-V characteristics based on the above-mentioned correlation. In this way finding the C-V characteristics of the wafer prior to fabricating the semiconductor circuit elements enables predicting the acceptability of backgate characteristics of semiconductor components such as FETs and HEMTs fabricated utilizing the wafer.
The foregoing backgate-characteristics determination device preferably is equipped with a display unit that displays a first C-V curve representing the first C-V characteristic and a second C-V curve representing the second C-V characteristic. Accordingly, acceptability of the backgate characteristics can be made by sight.
The foregoing backgate-characteristics determination device preferably is also equipped with a determination unit that determines, based on the first and the second C-V characteristics, the backgate characteristics of semiconductor circuit elements fabricated from the wafer. This allows determining the acceptability of the backgate characteristics without need for further personnel.
The foregoing determination unit preferably determines the backgate-effect characteristics based on a voltage-shift amount found by comparing a first C-V curve representing the first C-V characteristic and a second C-V curve representing the second C-V characteristic.
The abovementioned voltage-shift amount preferably is the difference between a voltage value that the first C-V curve represents for a predetermined capacitance value, and a voltage value that the second C-V curve represents for that capacitance value. The fact that in finding the second C-V characteristic a given voltage-shift amount can accordingly be found merely by seeking that voltage value at which the capacitance is a predetermined value enables the backgate characteristics to be readily determined.
Determining that the backgate-effect characteristics are unacceptable when the above-noted voltage-shift amount is a predetermined value or more, and that the backgate-effect characteristics are acceptable when it is less than the predetermined value, is preferable. Establishing a predetermined value for voltage-shift amount based on empirical data makes the determination standard clear, thereby making for enhanced reliability of the determination.
It preferable that for the above-noted first electrode, an electrolyte capable of etching the wafer be utilized as at least one voltage-applying terminal. C-V characteristics sought in a semiconductor layer in the wafer interior can be thereby be measured.